1. Field of the Invention
The present invention relates to an ultrasonic vibration apparatus such as an ultrasonic sensor used for detecting an object by transmitting and receiving ultrasonic waves.
2. Description of the Related Art
Hitherto, as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 8-15416, Japanese Unexamined Patent Application Publication No. 8-237795, Japanese Unexamined Patent Application Publication No. 9-284896, and Japanese Unexamined Patent Application Publication No. 10-257595, ultrasonic vibration apparatuses such as ultrasonic sensors employ a construction in which a piezoelectric element having an electrode formed on a piezoelectric plate is mounted in a casing.
Here, the basic construction of the ultrasonic vibration apparatus and an appearance of vibration thereof used for such conventional ultrasonic sensors are shown in FIGS. 9A and 9B. FIG. 9A is a cross sectional view showing a state in which a piezoelectric element 1 is mounted inside a casing 2. The casing 2 forms a cylindrical shape in which one end thereof serves as a disk-like vibration plate 2′ and in which the piezoelectric element 1 is bonded on the inner face of the end. When driving voltage is applied to the piezoelectric element 1, the piezoelectric element 1 conducts a bending vibration at a predetermined resonance frequency. Similarly, the disk-like vibration plate 2′ also conducts the bending vibration.
Thus, in the state in which the piezoelectric element 1 is bonded on the vibration plate, the resonance frequency depends on the material of the casing 2, the thickness a of the vibration plate 2′, and the diameter b thereof.
In such conventional ultrasonic vibration apparatuses, the sizes of the vibration plate 2′ influence not only the resonance frequency of but also the directivities of the ultrasonic waves at transmission time and at reception time. Generally, by widening the diameter of the vibration face and shortening the wavelength of the ultrasonic waves, directivity becomes narrowed. Accordingly, in an ultrasonic sensor in which narrow directivity is required, the outer diameter b of the casing is set to be large, further the thickness a is set to be great in order to set the resonance frequency to be high.
However, when the apparatus is used as an ultrasonic sensor, because of restriction in the size in the outer diameter and restriction of the wavelength to be used, the narrow directivity cannot be obtained without causing the apparatus to be large or without causing the operating frequency to be high.
Furthermore, the relationship that the directivity is determined by the area of the above-described vibration face and the wavelength is applied to, strictly speaking, a case in which the vibrating face is parallel-vibrating in a piston-movement manner and in which the ultrasonic wave is emitted as a plane wave. In the conventional ultrasonic apparatus in which the piezoelectric element is mounted in the cylindrical casing having simply one end thereof closed, since the vibration plate 2′ performs the bending vibration as shown in FIG. 9B, the ultrasonic waves propagate through air as a spherical wave front. Therefore, there is a problem in that little advantage in obtaining a narrow directivity is achieved even though the vibrating area is widened or the wavelength of the ultrasonic waves is shortened.
FIG. 10 shows the result of computation by a finite-element method (FEM) which is applied to the appearance of deformation in a vibration plate (the casing) due to vibration in a conventional ultrasonic vibration apparatus as shown in FIGS. 9A and 9B. FIG. 11 shows the result obtained by computing directivity characteristics of the ultrasonic waves which are emitted by this deformation. In this example, an angle (directivity angle) required to cause the sound pressure to be decreased up to −6.0 [dB], that is, to cause the sound pressure to be halved, is as wide as 44 degrees.